U.S. Pat. No. 4,089,464 discloses an instrument housing which includes expanders for telescopically receiving the flanges of the upper and lower cover members. U.S. Pat. No. 4,090,632 is also pertinent in that respect. A CIP of U.S. Pat. 4,089,464, Ser. No. 905,747, now U.S. Pat. No. 4,196,821, issued Apr. 8, 1980 includes a variation thereof whereby the side walls of the housing are formed by providing a pair of such expanders with a filler plate therebetween.